Printer or hickey-removing rollers (sometimes referred to as "hickey picker" rollers) are used in the offset printing industry to remove foreign particles, such as dirt, bits of paper or dried ink, called "hickeys" or "fisheyes", from the printing plate and inking train. Early hickey-removing rollers consisted of a heavy leather sleeve fitted over a hard (80 Shore A durometer) rubber-covered roller core. However, leather rollers were expensive and often took several weeks to manufacture. In use, leather hickey-removing rollers were difficult to clean, frequently needed to have the cover re-tightened and, after prolonged use, would eventually harden and scratch the printing plates.
In the prior art, U.S. Pat. No. 3,594,255 discloses a poromeric hickey-removing roller comprising a sleeve of felted fibers impregnated with a binding agent, such as polyurethane or polyurea, encasing a resilient elastomer and an underlying hard core. The sheet of polymer-impregnated felted fibers may be bonded to the roller by a flexible epoxy adhesive. Polytetrafluoroethylene (PTFE) particles are deposited in the surface pores of the impregnated poromeric sleeve by immersing the roller in an aqueous dispersion of PTFE particles. The PTFE particles are believed to have a beneficial effect due to the influence of electrostatic fields created when the roller is in operation.
During the operation of a printing press, the hickey-removing roller, which is in contact with the printing plate, turns at a high rate of speed. As ink splits between the hickey-removing roller and the printing plate, a high level of static electricity is produced. If the hickey-removing roller contains a repository for this static charge, the roller becomes strongly charged with an electrostatic charge of polarity opposite to that of the plate. Since the hickey particles are adhered to the plate, the hickey particles have the same charge as the plate. Therefore, the hickey particles are repelled by the charge of the plate and attracted to the oppositely charged hickey-removing roller.
Electret materials, such as fluorocarbon particles capable of maintaining a high charge of static electricity even in an aqueous or semi-aqueous environment, are excellent repositories for maintaining a static charge. By incorporating electret materials in the hickey-removing roller, hickey particles may be removed from the printing plate by attraction to the electret-containing materials of the hickey-removing roller.
Such a hickey-removing roller is described in U.S. Pat. No. 3,594,255, where the PTFE particles are the electret material. However, the electret particles of the disclosed hickey-removing roller are not uniformly dispersed within the pores of the poromeric polymer impregnated fibrous matrix covering the roller. During the PTFE impregnation process, the particles are only capable of entering the pores exposed at the surface of the roller. The poromeric covering of the roller is relatively rich in electrets at its outer surface, but comparatively starved below the surface. As the outer surface of the roller is abraded or worn away, the electret-rich layer ablates away leaving a roller surface having relatively fewer electrets and decreased electrostatic capabilities.
Another drawback of typical prior art hickey-removing rollers is the pattern in which the electrets are distributed. Because the electret impregnation process is only capable of depositing electret particles in the pores, the particles tend to clump in relatively small volumes of the poromeric surface.
In contrast with the prior art, the improved poromeric material of the present invention has electrets present substantially uniformly throughout the material so that as the outer surface of the material is abraded, the concentration of electrets in the exposed surface is substantially the same as that of the outer surface which was abraded away, thereby providing essentially consistent electrostatic properties throughout the poromeric material.